Sunburn is caused primarily by sunlight in what is known as the UVB band, or the wavelength range from 290 to 320 nm. The minimum dose of radiation needed to create a sunburn (erythema) is known as an MED (minimum erythema dose). The MED for a person of average resistance to burning is 50.0 mJ cm^–2.

a. Determine the number of 290-nm photons corresponding to the MED assuming each photon is absorbed. Repeat this calculation for 320-nm photons.

b. At 20° latitude, the solar flux in the UVB band at the surface of the Earth is 1.45 m W cm^–2. Assuming that each photon is absorbed, how long would a person with unprotected skin be able to stand in the sun before acquiring one MED?